The present invention pertains generally to electrical power dividers and specifically to a four-way power divider which employs 3db, 90.degree. couplers.
In electronics, quite often the processing of an electrical signal cannot be performed until the signal is first divided into two or more components. For example, in VHF-UHF applications a signal which might be in the order of 100 watts could require amplification to a much higher power level such as 400 watts prior to radio transmission. If a solid-state amplifier were to be used, as is now commonplace, then a single commercial transistor could not handle the amplification by itself since its rated capacity, which might typically be as high as 100 watts is much less than the total power imparted to the signal during amplification. This problem is obviated by the use of power dividers which divide the signal into smaller power components which can be individually handled by individual transistors during the signal processing and recombined into a single signal thereafter. In the foregoing example, a four-way power divider would be necessary in order to allow each 100 watt transistor of a four-transistor amplifier to amplify the signal after first dividing it into four power components. After amplification, the four signals would of course be recombined into one 400 watt signal.
A well-known element used in power dividers is the 3db, 90.degree. coupler (sometimes referred to as a quadrature hybrid coupler) which is discussed in a number of references including Chapter 13 , entitled "TEM-Mode, Coupled-Transmission-Line Directional Couplers, and Branch-Line Directional Couplers" of a book whose title is "Microwave Filters, Impedance-Matching Networks And Coupling Structures" by Matthaei, Young and Jones and an article "Don't Overspecify With Quad Hybrids" which appeared in two parts in the January and February, 1973, editions of Microwaves magazine. The 3db, 90.degree. coupler (coupler for short hereinafter) has two input ports and two output ports. With one input connected to a terminating impedance matched to the system characteristic impedance, a signal at the other input produces signals at the two outputs of the coupler each of which contains approximately one-half of the power engendered by the input signal (neglecting insertion loss) and differ in phase from each other by about 90.degree. . Because of the intrinsic design of the coupler, one output, conventionally referred to as the 0.degree. output (phase measured with respect to that of the input signal), contains more than one-half of the input signal power by a given amount and the other output, conventionally referred to as the -90.degree. output (phase likewise measured with respect to that of the input signal), contains less than one-half of the input signal power by the same aforementioned given amount over the operating frequency range of the coupler. If the output power at each port is normalized to the input power and measured in decibels (db), then this imbalance variation from a balanced condition, wherein both ports equally reflect one-half of the input power of -3db, typically extends to .+-.0.5db over the operating range so that the maximum output imbalance at the 0.degree. port is -2.5db and that at the -90.degree. port is -3.5db.
When one coupler is used to drive two other couplers in a triple coupler, four-way power divider, the imbalance deviation is doubled to + 1db. In this arrangement, two of the four power divider outputs are balanced at -6db (one quarter of the input signal power) over the operating frequency range, but the other two outputs are unbalanced with the 0.degree. output exhibiting -5db and the -90.degree. output exhibiting -7db. Since economical design considerations dictate that like functions in an electrical device such as a power amplifier be performed with like components, a consequence of the foregoing imbalance is that the full capacity of a four transistor amplifier can never be realized. Minus 5db at the 0.degree. output of the four-way divider corresponds to about 32% of the input signal power (as opposed to the desired 25% for a balanced condition), so that when operating at its full capacity the other three transistors would be delivering only 68% of the input power although having a combined power rating of 95% of the input power (assuming the use of four equally rated transistors). Thus, despite the fact that the four transistor amplifier is capable of delivering 128% of the input signal power if operated in a balanced condition with each transistor carrying one-quarter of the load (32% of input power), more than 20% of its balanced rated capacity must go unutilized in order to avoid exceeding the capacity of the one transistor which would be connected to the 0.degree. output of the power divider when operating in an unbalanced fashion.
With the foregoing in mind, it is a primary object of the present invention to provide a new and improved four-way power divider which employs 3db, 90.degree. couplers.
It is therefore an object of the present invention to provide such a new and improved four-way divider which exhibits no output power imbalance as heretofore.
The foregoing objects as well as others and the means by which they are achieved by the present invention may be best appreciated by referring to the Detailed Description of the Invention which follows together with the appended drawings.